One sobering reality we’ve all learned in the aftermath of the terrorist attacks of 9/11 is that a high-rise building may be susceptible to structural collapse. All three buildings attacked that day were hit by “flying bombs,” loaded with fuel that instantly ignited into sizable high-challenge infernos.
We saw how the Twin Towers were reduced to “ground zero.” Thirty-five minutes after the second plane hit in New York, a Boeing 757 flew low over Arlington, Va., at more than 500 miles an hour and crashed into the first and second floors of the Pentagon, a decidedly low-rise building, in fact, the largest low-rise building in the world.
Consider the 102-story Empire State Building, with its 2.2 million square feet of floor space. The five-story Pentagon contains more than three times that amount. It’s hard to comprehend its mammoth size: 17 miles of corridors, 131 stairways, 672 fire hose cabinets, 7,754 windows, and a 600,000 square-foot basement slab.
The structural system of the famous five-sided building consists entirely of cast-in-place reinforced concrete using normal-weight aggregate. Columns support floors constructed as a slab, beam and girder system.
The explosion of the plane’s jet fuel created a fireball as the fuselage plunged in, destroying or significantly weakening 50 structural columns. The upper floors in the area of impact collapsed into the debris within 20 minutes.
However, when it was over, the Pentagon lost less than 4 percent of its total area of operation. While 125 Department of Defense workers were killed, they likely died instantly as a direct effect of the collision. More than 24,000 people worked in the building at the time.
There are several reasons why the building, its staff and functional ability survived as well as they did.
To best determine optimum disaster response procedures, the Pentagon had even run a fire drill earlier in 2001, which simulated a plane crash into the building’s center court area. A security analyst for this practice run reported that the drill “probably saved hundreds of lives ... everyone on site knew how to react logistically, if not emotionally.”
The disaster also was limited by a recent renovation/modernization, the nature of fire and the emergency response.
Ironically, construction of the building began on Sept. 11, 1941. The Pentagon answered the War Department’s need for office space and a centralized hub for operations during World War II.
But high tech it wasn’t. So an extensive modernization project began in 1999 to renovate the electrical as well as the mechanical systems.
With newly installed raised flooring, the computer-intensive office operations were relocated to a new mezzanine level, with computer support in the basement. (This information had been previously disclosed, leading some to speculate that was the reason the terrorist flew the plane in low, as opposed to striking the roof.)
Also relocated were the four main incoming utility circuits, carefully designed to avoid any interruption or power failure to key electrical units. New network transformer vaults were constructed, boosting total system capacity. The basement was retrofitted with suitable drainage systems, and a central air-handling system was modified for two entire levels.
Engineers knew the building had to be kept in continuous operation without disrupting its information management and telecommunication functions and other day-to-day requirements. Some of the mechanical components were modified so that they could remain working in a portable mode.
On 9/11, about a year of work remained on the renovation. The plane struck the first floor in the renovated portion of the building approximately 140 feet to the south of the end boundary of the renovated section.
Horizontal, Not Vertical
Fire burns upwards and outwards. Outwards, depending on the combustible fuel load present, it burns steadily, but at a comparably slow pace. Upwards, however, it defies gravity and incinerates all in its path, gaining speed like a flamethrower.
One way to understand this is to observe any fire and notice that the visible flames reach upward. The released heat will travel to the highest point in the area. So while all fires take on different profiles, they almost always travel faster in a vertical direction.
This is why it takes longer for a “horizontal” building to become a total loss. The fuel explosion certainly did accelerate the outward growth of the Pentagon fire. But after the initial blast, the fire development was nothing like that experienced in the Twin Towers. With a massive amount of compartmentalized fire-retardant construction destroyed, as well as the sprinkler systems, those fires were free to instantaneously mount vertically at great magnitude.
It’s obvious in a tragedy of this magnitude that a building structure is under stress from intense heat as well as impact damage. But for the most part, the Pentagon stood safely. Less steel and more concrete generally means more impediments to fire growth. The fortress-like concrete and limestone construction of the Pentagon is not conducive to the typical growth rate of a fire that will geometrically double in intensity by the minute.
Concrete is actually used as a protective covering for other materials, such as steel. It impedes convection of heat. In addition, a considerable amount of the heat energy of a fire is expended in vaporizing the concrete’s moisture content. Thus the structural integrity of a building will be more effectively maintained until the fire department can arrive on the scene.
A study of the attack made by the American Society of Civil Engineers reported that damage to the robust structure was “mitigated by the Pentagon’s resilient structural system” and “survived this extraordinary event better than would be expected.”
The Army Corps of Engineer’s study cited closely spaced spiral reinforcement in the lower-story columns as being responsible for the energy-absorbing capacity of the strong reinforced concrete frame.
The Army Corps’ summary concluded with a recommendation that “the features of the Pentagon’s design that contributed to the resiliency in the crash, be incorporated in the future into the design of buildings in which resistance to progressive collapse is deemed important.”
A typical commercial building does not come equipped with the exhaustive security and alarm measures present throughout the Pentagon.
The first responders to the emergency were the Arlington County firefighters, all of who arrived on the scene that morning.
With sprinkler systems demolished, firefighters managed to strategically contain the blaze. With little margin for error, they doused the fire completely less than eight hours after the initial explosion, allowing key Pentagon officials to continue their necessary defense work in other portions of the building.
The firefighters also had a few factors in their favor:
- Most importantly, the fire was accessible to firefighting teams using equipment from the ground.
- Flight 77, leaving Dulles at 8:21 a.m. and bound for Los Angeles, carried roughly 5,300 gallons of fuel compared to the 24,000 gallons carried by the Boeing 767s that rammed into the World Trade Center. The resulting explosion packed less than half the punch that burst in each tower.
- Since the Pentagon fire was mainly in an “open” area, radiant heat could quickly escape into the atmosphere (instead of radiating into adjacent building areas), eliminating flashover potential.
- Another factor that played a role in mitigating flame spread that day was the fact that there was very little wind.
The Pentagon fire was contained after spreading 50 feet to either side of the 100-foot impact area on the 921-foot exterior of the southwest “fifth,” not quite reaching the center courtyard, and spreading up to the third floor.
The fire did keep rekindling for the next three days as rubble removal exposed still-smoldering materials. However, these small fires were easily extinguished. Luckily, there were no firefighter fatalities in Washington, D.C.